Crystal Structures and Intermolecular Interactions in α$\alpha$‐ and β$\beta$‐phosgene

The crystal structures of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>α</mml:mi> <mml:annotation>$\alpha$</mml:annotation></mml:semantics> </mml:math> - and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>β</mml:mi> <mml:annotation>$\beta$</mml:annotation></mml:semantics> </mml:math> -phosgene have been elucidated on the basis of variable temperature neutron powder diffraction. Although the stable <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>α</mml:mi> <mml:annotation>$\alpha$</mml:annotation></mml:semantics> </mml:math> -phase closely mirrors the published structure ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>I</mml:mi> <mml:msub><mml:mn>4</mml:mn> <mml:mn>1</mml:mn></mml:msub> <mml:mo>/</mml:mo> <mml:mi>a</mml:mi></mml:mrow> <mml:annotation>$I4_1/a$</mml:annotation></mml:semantics> </mml:math> , No. 88), the new metastable <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>β</mml:mi> <mml:annotation>$\beta$</mml:annotation></mml:semantics> </mml:math> -phase crystallises in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>C</mml:mi> <mml:mi>m</mml:mi> <mml:mi>c</mml:mi> <mml:msub><mml:mn>2</mml:mn> <mml:mn>1</mml:mn></mml:msub> </mml:mrow> <mml:annotation>$Cmc2_1$</mml:annotation></mml:semantics> </mml:math> (No. 36, at 135 K: a = 10.244042(22) Å, b = 6.280321(21) Å, c = 5.46069(4) Å). Both modifications show dipole-dipole interactions, but only <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>α</mml:mi> <mml:annotation>$\alpha$</mml:annotation></mml:semantics> </mml:math> -phosgene shows pronounced <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>σ</mml:mi> <mml:annotation>$\sigma$</mml:annotation></mml:semantics> </mml:math> -hole interactions. Quantum chemical calculations in both solid state and of model dimers in gas-phase reveal a complex interplay of intermolecular interactions. The first inelastic neutron scattering spectra of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>α</mml:mi> <mml:annotation>$\alpha$</mml:annotation></mml:semantics> </mml:math> -phosgene shows an unusual separation of the translational and librational modes, alongside sharp bands for the internal modes. Our computational studies also show an unusual, extensive mixing of the in-phase C-Cl stretch ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>ν</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:annotation>$\nu _2$</mml:annotation></mml:semantics> </mml:math> ) and the out-of-plane bend ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>ν</mml:mi> <mml:mn>6</mml:mn></mml:msub> <mml:annotation>$\nu _6$</mml:annotation></mml:semantics> </mml:math> ) modes, which are symmetry-forbidden from mixing in the gas-phase.